Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 4
Presentation Time: 9:15 AM


BRIGHT, Ryan M.1, AMATO, Jeffrey M.1, DENYSZYN, Steven W.2 and ERNST, Richard E.3, (1)Geological Sciences, New Mexico State University, P.O. Box 30001/MSC 3AB, Las Cruces, NM 88003, (2)School of Earth and Environment, University of Western Australia, 35 Stirling Highway M004, Perth, 6009, Australia, (3)Dept. of Geology, Carleton University and Ernst Geosciences, 43 Margrave Avenue, Ottawa, ON K1T 3Y2, Canada,

The widespread Mesoproterozoic (ca. 1.1 Ga) Southwestern U.S. Diabase Province consisting of dikes, sills, and sheets, is located in southwestern Laurentia. Southwestern U.S. diabase has low to moderate whole rock Mg# (24–64) and concentrations of CaO and Al2O3 that suggest fractionation of olivine and plagioclase occurred before emplacement. Total alkali concentrations (Na2O + K2O) ranging from 2–5 wt.% indicate that diabase is transitional from tholeiitic to alkaline. Enrichments of large ion lithophile elements relative to mid-ocean ridge basalts and negative Nb anomalies relative to ocean island basalts suggest that diabase was derived from a weakly enriched mantle source. This is supported by whole rock εNd values of 3.2 to -0.5 that correlate with 87Sr/86Sri ratios for whole rock and single crystal plagioclases ranging from 0.694 – 0.710. Some plagioclase crystals have higher 87Sr/86Sri ratios than the corresponding whole rock samples, indicating assimilation of Proterozoic country rock occurred. Xenocrystic zircon in some samples provides further evidence for assimilation. We obtained a U-Pb ID-TIMS age on baddeleyite (ZrO2) from a diabase dike in the Pinaleño Mountains, Arizona, of 1087 ± 6 Ma (2σ), using the weighted mean 207Pb/206Pb age of 6 fractions. U-Pb SHRIMP dating of zircon from a diabase sill in the Salt River Canyon, Arizona yielded a weighted mean 207Pb/206Pb age of 1088 ± 11 Ma (2σ; 7 spots). Northwest-striking dikes in Arizona indicate extension occurred perpendicular to Grenville shortening. However, structural and geochemical relationships suggest a mantle plume created mantle-derived magmas that intruded into a faulted upper crust. The age of the Southwestern U.S. Diabase Province is similar to that of the Warakurna Large Igneous Province in Australia (~1075 Ma), supporting a close fit between Laurentia and Australia at this time. The projected intersection of the NW-SE trending dike swarm of the Diabase Province with the western margin of Laurentia may represent a piercing point that constrains the position of Australia along western Laurentia in the Rodinia supercontinent.